Inch of mercury

Inch of mercury (inHg or "Hg) is a non-SI unit for pressure, or a way to measure how hard something is pushing on something else. It is a unit that is still used a lot for counting barometric pressure, or how thick the air is in one place, in weather forecasts and aviation, or anything that has to do with flying in the United States, but is considered a bit old in other countries.

Airplanes that are flying at high altitudes, or that are flying higher than what is called the Transition Altitude, which varies by country set their barometric altimeters, which control the air pressure in their plane, to a standard pressure of 29.921 inHg (101.325 kilopascals) no matter what the actual sea level pressure is, in inches of mercury used in the U.S. and Canada, and in other units in other countries. The resulting altimeter readings, or the things that show up on the plane's altimeter are known as flight levels.